1. Field of the Invention
The invention is related to the field of wellbore drilling. More specifically, the invention is related to tools used in back reaming operations, such as used to create boreholes river crossing and similar horizontal drilling applications.
2. Background Art
Horizontal directional drilling (HDD) is a technique used to create subsurface conduits underneath roadways, river beds or other obstructions in the path of things such as petroleum product pipelines and communication cable passageways.
Typically, a specialized drilling rig, such as one sold under the trade name DITCH WITCH by the Charles Machine Works, Inc. Perry, Okla., is used to drill the subsurface conduits. An entry hole is bored at the earth's surface on one side of the obstruction, using a steerable drilling head attached to one end of a drill string. The drill string is generally made of a number of segments or “joints” of threadedly coupled drill pipe. The entry hole is started at an angle slightly inclined from horizontal so that the conduit will become increasingly deeper in the ground as the conduit extends laterally away from the surface position of the entry hole. When the conduit reaches a sufficient depth, the conduit is drilled substantially horizontally until it crosses the lateral surface position of the obstruction. Then drilling proceeds in a slightly upward direction, continuing laterally away from the obstruction, to terminate the conduit at an exit hole on the earth's surface on the other side of the obstruction.
To complete the conduit, a service cable or pipe is attached to the exposed end of the drill string at the exit hole, and is pulled back to the drilling rig along with the drill string. Often, the conduit driller or operator may wish to increase the diameter from that initially drilled during the directional drilling operation. A device known as a back reaming tool is coupled to the end of the drill string to perform this enlargement as the drill string is withdrawn from the conduit. Several different types of back reaming tools are known in the art.
A first type of back reaming tool is formed from a roller cone drill bit of a type used to initially drill the conduit, or of a type used in petroleum and mining wellbore drilling operations. In such roller cone bit type back reaming tools, roller cones are disposed so that their cutting ends face the drilling rig from the exit hole. As the drill string is withdrawn from the conduit, the drill string is rotated so that roller cones on the back reaming tool will cut the walls of the conduit to enlarge the conduit diameter. Drill bit type back reaming tools are essentially an improvisation, and while they have proven commercially successful, they have limited application because of the difficulty in making them and the fact that once any of the cutting elements, any one of the roller cones, or any of the rotary bearing structures on the roller cones wear out or fail, the entire reaming tool must be replaced.
Another type of back reaming tool is intentionally designed as a back reaming tool, and includes a reaming tool body, to which are removably attached a plurality of cutting structures. Each one of the cutting structures includes a roller cone rotatably mounted on a bearing journal. In one embodiment of a back reaming tool known in the art, the bearing journal is removably mounted at both ends thereof in a cradle. The cradle is removably mounted to the tool body. In another embodiment of a back reaming tool known in the art, the bearing journal is threadedly coupled at one end to the cradle. A common aspect of the back reaming tools known in the art is that they include roller cone cutting structures which are exposed to wellbore fluids at both axial ends. Therefore, the back reaming tools known in the art require that the bearings be sealed in two places along the axis of the bearing journal to exclude wellbore fluids and maintain adequate bearing life. Another aspect common to back reaming tools known in the art is that they include a plurality of roller cones rotatably mounted on the tool body. Limitations on the minimum useful size of the bearing journal limits the diameter of conduits which may use such back reaming tools. Another aspect common to back reaming tools known in the art is that they use roller cones for the cutting elements thereon.